1. Field of the Invention
This invention relates generally to bending pipe, and specifically to an apparatus for bending heated, pre-cut lengths of thermoplastic, particularly polyvinyl chloride (PVC), pipe into elbows.
2. Description of the Prior Art
Elbows made from PVC pipe have many uses, including as conduits to string electrical wires through when wiring residential, commercial, or other buildings, or when introducing wire underground. A PVC elbow ordinarily has pipe coupled at each end to complete the wiring job. Depending upon the user's specifications to complete a particular job, PVC elbows are usually supplied having an inside diameter ranging between 0.5 inches to over 6 inches and a specified radius of curvature. PVC elbows of this type are generally required to have a degree of bend as small as 5.degree. but not to exceed 90.degree., with a tolerance of about plus or minus 2.degree..
Apparatus for bending PVC pipe are known in the art. U.S. Pat. No. 3,890,079 issued to Slater discloses an automatic machine for bending and then flaring the ends of plastic pipe. This is accomplished by a two-piece die between which a straight pipe is placed. One piece of the die is mechanically translated toward the other stationary piece thereby bending the pipe into an elbow. Injection plugs are inserted in each end of the elbow thereby flaring the ends. Air is injected into the pipe to internally support it thereby preventing any significant deformations. Once the pipe has been shaped, the injection plugs inject water into the pipe, and the pipe is dropped into a tank of coolant.
Another apparatus for bending thermoplastic pipe is described in U.S. Pat. No. 3,884,612 issued to Parmann. Parmann discloses a method and apparatus for bending thermoplastic pipe wherein the steps are carried out at a single location while one end of the pipe is held in a fixed position. The pipe is first heated internally by a hot compressed air stream, and then a rubber container is lowered into the pipe so that pressure can be exerted against the interior walls of the pipe. The hot compressed air stream can be applied at various angles and pressures depending upon which part of the pipe is to be fashioned. Both aforementioned patents require that the pipe be internally supported during the bending operations.
Another prior art technique for bending PVC pipe is to heat a length of pipe and place it into a hand-held bending form. The bending form includes an elbow shaped die to which the heated pipe conforms when bent. One end of the elbow shaped die has an extrusion over which one end of the heated pipe is placed. A spring is inserted in the pipe so the pipe will maintain its shape while bending. A worker bends the heated pipe by hand until it is formed against the elbow shaped die. A plug hinged to the other end of the die is inserted into the pipe to hold the pipe against the die while cooling. The bending form is then submersed into a water bath and held until the PVC pipe cools sufficiently to retain the elbow shape. Once the elbow is cooled, the worker removes the bending form from the tank, removes the hinged plug and the spring from the elbow, removes the elbow from the bending form, and places the elbow into the water bath to assure complete cooling. The worker then gets another heated length of pipe and repeats the entire procedure. This procedure is disadvantageous because it requires the worker to perform several steps on the work piece and is time consuming. Furthermore, because the process is performed substantially by hand workers may experience fatigue in their hands and therefore require periodic rests.
Additionally, the elbow shaped dies which form part of the hand-held bending form have been constructed from metals, such as aluminum, and from plastics. It has been discovered that a plastic die having a low thermal conductivity allows proper cooling of the elbows. Metal dies allow too much heat to transfer from the heated length of pipe to the die which can lead to an improper cooling of the elbow. Improper cooling often results in the skin of the elbow becoming wrinkled or deformations occurring throughout the wall thickness of the pipe.